Apparatus for the catalytic conversion of waste gases containing hydrocarbon, halogenated hydrocarbon and carbon monoxide

ABSTRACT

Apparatus for carrying out a method for the catalytic conversion of waste gases such as those which accumulate in the synthesis of vinyl chloride is presented in which the waste gases are successively treated at an elevated temperature with a catalyst for oxidative cracking and with a catalyst for oxidative afterburning.

REFERENCE TO A RELATED APPLICATION

This application is a continuation of application Ser. No. 07/826,591, filed Jan. 28, 1992, and now abandoned, which application is a continuation-in-part of application Ser. No. 07/595,845, filed on Oct. 11, 1990, and now U.S. Pat. No. 5,108,717, which application is a divisional of application Ser. No. 247,297, filed Sep. 21, 1988, and now U.S. Pat. No. 4,983,366, which applications are herein incorporated by reference.

INTRODUCTION AND BACKGROUND

The present invention relates to an apparatus for the catalytic conversion of waste gases containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, especially from syntheses carried out in the preparation of vinyl chloride.

In addition to the increasing pollution of the air by SO₂ and NO_(X), the emission of hydrocarbons and especially halogenated hydrocarbons has become a serious problem. Thus, for example, waste gases accumulate in preparation processes carried out for making vinyl chloride which contains, in part, extremely noxious substances. Compliance with applicable regulations pertaining to air quality is best achieved by means of an additional cleaning of waste gases.

Of the great number of known measures for reducing emissions of substances which pollute the air, thermal or catalytic cleaning methods are those primarily used to this end since especially high levels of cleaning effect are achieved with them.

Thus for example, in thermal afterburning, the pollutants to be eliminated are oxidized under conditions supplying energy at temperatures up to 1500° C. in a combustion chamber ("VDI-Proceedings" 525, Catalytic and Thermal Methods of Cleaning Waste Gases, Meeting, April 7-8, 1985, VDI-Verlag, Duesseldorf, 1985, p. 347).

The residence time in the combustion chamber is at least 1 second (VDI loc. cit., p. 347). In accordance with economic considerations, thermal cleaning is only advantageous in the case of high concentrations of pollutants. A considerable disadvantage resides in the costs for the generation and maintenance of the conversion temperatures as well as in the fact that the heat energy released during the afterburning process can not always be utilized in an economical fashion.

Catalytic afterburning is usually performed with the aid of noble metal catalysts at approximately 300°-700° C. and with residence times up to 0.3 seconds. It is to be preferred in the case of low concentrations of pollutants and is distinguished in comparison to the thermal method by distinctly lower operating costs (VDI loc. cit., p. 75). However, there are limitations when using the catalytic method, based on a possible reduction of activity of the catalyst due to too high thermal stress or chemical deactivation by means of catalytic poisons present in the waste gas (VDI loc.cit., p. 75).

When using the more economical catalytic cleaning of waste gas resulting from the preparation processes used for making vinyl chloride, the problem confronting the industry involved keeping the thermal stress of the catalyst low by means of chemical techniques, thus counteracting a poisoning effect as much as possible.

SUMMARY OF THE INVENTION

An object of the invention is to provide apparatus to carry out a method for the catalytic conversion of waste gases containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, especially those obtained from the synthesis of vinyl chloride. In achieving the above and other objects, one feature of the invention resides in apparatus for carrying out a first step of passing the waste gases at 300°-800° C., preferably 350°-730° C. through a first zone containing a catalyst for oxidative cracking and then through a second zone containing a catalyst for oxidative afterburning. The entrance temperature of the waste gas must be above the ignition temperature of the catalyst. In order to achieve sufficient conversion, the waste gas temperature immediately prior to entry into the first zone should not be under 300° C. and not exceed 800° C. in both zones in order to avoid thermal damage to the catalyst. Oxidative cracking of rather long-chain, aromatic-aliphatic hydrocarbons (1) (see equation below) into smaller molecules, oxidative afterburning of unsaturated and saturated hydrocarbons and halogenated hydrocarbons (2) and oxidation of CO to CO₂ (3) take place in the catalytic zones according to the following: ##STR1##

Basically, all catalysts indicated in the literature and customary for the catalytic purification of waste gases can be used to this end. Thus invention can be carried out with desired combinations of catalysts as is also known from the literature.

In general, all catalytic systems known for oxidative cracking of organic compounds can be used in the first zone of the apparatus of the invention. In the instant, special case of the catalytic conversion of waste gases containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, especially waste gases obtained from the synthesis of vinyl chloride, it has proven to be very important to use a catalyst which brings about only a partial conversion. In particular, the reactions which occur here in the presence of an excess of air are oxidative decompositions, dehalogenations and dehydrohalogenations. Impurities such as aliphatic and aromatic hydrocarbons and chlorinated hydrocarbons such as ethyl chloride, vinyl chloride and 1,2-dichloroethane, etc. are converted to a large extent whereas substances such as methyl chloride and methane are converted only partially and carbon monoxide practically not at all.

A considerable portion of the reaction heat is already released in the first zone by the preliminary conversion there so that temperature peaks (with an associated sintering of carrier material and catalytically active phase) are avoided to a large extent in the following second zone of the very active second catalyst; this has an advantageous effect on the service life of this catalyst.

The relatively insensitive catalyst of the first zone functions, in addition, as a filter for heavy metals from apparatuses and pipelines and for other catalyst poisons. As a result of its surface quality, it exhibits the property of catching the Cl atoms produced in situ during the reaction on the surface and thus protecting the subsequent sensitive catalyst of the second zone to a large extent from poisonings. Especially preferred, catalytically active substances for the catalyst of the first zone are aluminum oxide, silicon dioxide, aluminum silicate and/or a zeolite, optionally with a content of 0.1-20% by weight of oxidic compounds of one or more of the elements Ba, Mg, Cu, Cr, Mn, Ni.

The catalyst of the second zone is used to accomplish as complete as possible a conversion of the thermally stable hydrocarbons and halogenated hydrocarbons as well as for the oxidation of carbon monoxide. It must therefore furnish a good oxidation action. Fundamentally, base metal catalysts or solid contacts as well as noble metal catalysts can be used here. Especially good results are obtained if the catalytically active substance of the catalyst of the second zone is platinum and/or palladium or platinum and rhodium. In these instances, platinum and palladium can be present in a weight ratio of 1:5-5:1 and platinum and rhodium in a weight ratio of 5:1-20:1.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further understood with reference to the accompanying drawings, wherein:

FIG. 1 shows a schematic elevational cross section view of an embodiment of the apparatus of the present invention.

and FIG. 2 shows a schematic elevational cross section view of a second embodiment of the apparatus of the present invention.

DETAILED DESCRIPTION OF INVENTION

Many varied embodiments can be used for the catalysts of the first and/or second zone. It has proven to be advantageous if the catalyst of the first and/or second zone is formed as non-shaped or shaped bulk material made of the catalytic substance or in the form of carrier bodies provided with a coating of the catalytically active component, or as monolith coated with a carrier material for the catalytically active component which carrier material promotes catalysis, or as monolith provided with the catalytically active component, or as monolith of material which promotes catalysis, or as monolith of the catalytic material itself.

The non-shaped or shaped bodies of bulk material can be present e.g. as granulate, pellets, balls, tablets, extrudate, extrusion compacts briquettes, hollow billets, saddles, grids or in any shaped embodiments which are provided with the catalytically active substance by immersion, dipping, spray impregnation, sputtering or according to any other generally customary methods. If porous carrier material is used, the catalytically active components can be evenly distributed in the entire shaped body or, preferably, concentrated on the outer surface. Such a so-called shell catalyst can also comprise an inert core of solid material such as e.g. μ-Al₂ O₃ or cordierite, e.g. in ball form or as honeycomb body, for structural reinforcement.

As a result of the mode of operation of both catalytic layers, which are coordinated with each other in a purposeful manner, the operable life-times are achieved even with the more sensitive noble-metal catalyst of the second zone which were not possible previously in traditional single-bed methods with such catalysts.

A particularly advantageous embodiment of the apparatus of the invention provides for arranging two catalysts as a double bed or at a distance in spaced relationships from one another. The double-bed design is particularly space-saving and therefore associated with the least equipment costs. On the other hand, the arranging of the zones at a distance from one another assures a more even temperature distribution brought about by means of vortexing of the waste gas prior to entry into the second zone. In addition, a certain cooling of the gas (e.g. by feeding in air) can be brought about with this arrangement, especially at a high heat value of the waste gas, in order to protect the catalyst of the second zone from overheating. The distance between the two zones can be selected as desired and is limited as a maximum spacing only by the dimensions of the equipment.

In order to assure an as large as possible contact area for the gas flow in the available inner area of the reactor, the waste gas is guided according to a preferred embodiment of the invention through an annular arrangement of the two catalytic zones. This annular arrangement is cylindrical. The direction of flow can be selected so that the waste gas is guided through the cylindrical arrangement from the outside toward the inside or vice versa. The catalytic zones can be designed as a bed of bulk material and/or a monolithic arrangement. The monoliths can consist of ceramics, or conventional metal honeycombs can also be used. If monolithic or fixed-bed catalysts or a combination of these catalysts is used, the zones can be arranged in the form of a polygon. Basically, polygons beginning with the form of a triangle up to an octagon (including all forms in between) are possible as the base area; in all these cases the annular space can be retained.

The heat generated during the catalytic conversion can be used to preheat the waste gas to be cleaned in a heat exchanger heated by the cleaned waste gas.

Since chemical reactions of mixtures of substances exhibit differing activation energy, in order to achieve the starting temperature of the less active catalyst, which comes into contact with the waste gas first (that of the first layer), the waste gas to be cleaned must usually be temporarily heated by external heating to the ignition temperature of the catalyst, in this specific application approximately 400° C., when the reactor is started. Possible types of external heating are e.g. gas burners, IR radiators, ignition by means of electrode (via generation and spreading of a heat envelope) or other energy sources. If the heating value of the waste gas to be converted is sufficiently high, the reactor can also be operated without external heating after the ignition temperature has been achieved along with an appropriate guiding of the waste gas. The pressure conditions in the combustion of waste gas are not critical. The conversion can be performed at normal pressure to 10 bars excess pressure.

In the first apparatus embodiment of the present invention there is provided a simple and purposeful apparatus for carrying out the method of the invention. This apparatus, schematically shown in elevational cross section in FIG. 1, includes a cylindrical reactor housing (1) with a circular opening (3) defined by an annular bottom (2), an arrangement of three cylinders (7),(8),(9) with perforated jackets which cylinders are placed at intervals according to the thickness of two annular layers (5,6) of bulk catalytic material to be received therein (not shown). The cylinders rest on the annular bottom in a gas tight manner and are closed at the top by a plate (4). The first inner jacket (7) of the group of jackets is aligned with the interior most edge (10) of bottom opening (3). The second jacket (8) of the arrangement separates the two catalytic layers. The outer jacket (9) of the illustrated arrangement is at a uniform distance or a distance which widens out in an upward direction. The annular space (11) is thereby formed between the cylinders and the wall of the reactor housing.

A feature of the apparatus is a mixing and preheating zone (13) adjacent to the bottom of the reactor housing, below the circular opening (3), and communicating with the secondary, or downstream, side of heat exchanger (12). Connected into the zone (13) line (14) for the supply of a gaseous heating medium which line empties laterally into zone (13). Waste gas outlet line (16) communicates with the upper head area (15) of the reactor housing. Line (16) runs to shunt or branching point (17) from which a branch line (18) communicates with the inlet connection piece of the heat exchanger for the heating medium near or at the downstream end of the heat exchangers. The other branch line (18) communicates via closing member (19) with waste gas discharge (20) of the apparatus. The closing member can be any suitable flap or single valve. The outlet connection piece of the heat exchanger for the heating medium at or near the upstream end in another feature of the invention communicates via line (21) directly with waste gas discharge (20) and the primary side or upstream opening of the heat exchanger is connected to supply line (22) for the waste gas to be converted.

The apparatus described above can be varied in such a manner that the two catalytic layers consist of superimposed monolithic catalysts and/or bulk material catalysts. Thus the cylindrical holder arrangement is replaced with replacement of cylinders (7,8,9) by a polygonal arrangement of known holder devices for monoliths or fixed beds.

If it is desired that the preheating of the waste gas to be converted to ignition temperature be performed exclusively in the heat exchanger, the apparatus can be varied in such a manner that the line for the supply of the gaseous heating medium does not empty in zone (13) but rather in head room (15) of the reactor housing.

The operation of the apparatus shown by way of example in FIG. 1 is as follows:

The waste gas to be converted enters at entry port (22) into heat exchanger (12), then passes into mixing and preheating zone (13) where it is preheated to the required starting temperature of the catalyst by means of a gaseous heating medium supplied via line (14). The heating medium is produced in a burner arrangement by burning a mixture of methane and air. If required, secondary air can be added after the burner for afterburning.

The preheated waste gas then passes successively through catalytic layers (6,5). The catalyst to be located in space (6) is a catalyst for oxidative cracking and a noble metal oxidation catalyst for oxidation afterburning is in location space (5). The converted waste gas passes through the perforated walls of the jackets and leaves the reactor via annular space (11), head space (15) and line (16). The converted waste gas can then be supplied entirely or partially by the appropriate positioning of flap (19) via line (18) as heating medium to the heat exchanger. Annular space (11) widens out conically upward in the apparatus design shown, thus creating improved flow conditions for the cleaned waste gas passing along the catalytic arrangement with volumetric increase into annular space (11).

If there is high exothermicity in the reactor, the recycling of waste gas to be downstream inlet side of the heat exchanger can be practically eliminated. The cleaned waste gas cooled down in heat exchanger (12) finally passes via line (21) directly to waste gas discharge (20) of the apparatus. This current of waste gas can be fed to a further heat recovery system for additional economics.

In yet another aspect of the present invention there is provided a second apparatus for carrying out the method of the invention. This apparatus is schematically shown in FIG. 2. It is an apparatus for purifying waste gases comprising a cylindrical reactor housing (1'), a heat exchanger (12'), a supply pipe (22') for the untreated waste gas and an exhaust pipe (20') for the cleaned waste gas. Said heat exchanger (12') has primary passageways (23') for conveying the cleaned waste gas and secondary passageways (24') for conveying the as yet untreated waste gas which are in close thermal contact with each other. The reactor housing (1') has an upper portion forming a head chamber (15') and a lower portion forming a heating chamber (25') which is connected via the primary passageways (23') of said heat exchanger (12') with said exhaust pipe (20'). The untreated waste gas supply pipe (22') is connected with the inlet port (26') of said secondary passageways (24') while the outlet port (27') of the exchanger is connected via another pipe (28') with said head chamber (15'). The secondary passageways (24') are bypassed by a shunt pipe (29') running from said supply pipe (22') directly to said another pipe (28'), wherein said shunt pipe (29') has a throttling member (30'). The reactor housing (1'), including a catalytic section, is separated from said heating chamber (25') by an annular bottom (2') having a central hole (3') with a circumferential edge (10'). The reactor housing (1') includes an arrangement of two coaxial annular cages (5',6') for receiving bulk catalytic material so as to form two catalytic layers. The annular cages (5',6') are formed by three conical wall members (7',8', 9') including an inner wall (7'), a separating wall (8') and an outer wall (9'). The conical wall members are perforated and rest with their bases on said annular bottom (2') of the reactor housing (1') catalytic section and are closed at the top by a common plate (4') The conical wall members (7', 8', 9') are sealed to said annular bottom (2') in a gas tight manner whereby said inner wall (7') forms a circumferential border on said annular bottom (2') which coincides with said circumferential edge (10') of said central hole (3'). The reactor housing (1') interior wall and said outer wall (9') of the conical wall member form an annular space (11') with increasing cross section from the bottom to the top of said housing (1') catalytic section due to the decreasing diameter of said outer wall (9'). The heating chamber (25') is laterally connected to a pipe (14') for the supply of a gaseous heating medium. The common plate (4') at the top of the conical wall members closes the central hole to force the waste gas entering the headchamber (15') and coming into the top of said annular space (11') and through the catalyst materials in space 5', 6' and through said central hole (3'). The treated waste gas then enters the primary passageways (23') of said heat exchanger (12') and flows towards said exhaust pipe (20') exiting the apparatus.

The two coaxial annular cages (5',6') for the uptake of the catalytic active material can, as in the case of the first apparatus, be replaced by a polygonal arrangement of known holder devices for monoliths or fixed beds.

The described second apparatus can be preferably used for the purification of waste gases loaded only to a small extent, whereby because of the missing reaction heat burning gases are to be introduced continuously.

The second apparatus illustrated in FIG. 2 operates as follows:

The waste gas to be reacted is introduced into the apparatus via a supply pipe (22'); it flows through the secondary passageways (24') of the heat exchanger (12') and thereafter passes through the pipeline (28') to the head chamber (15') of the reactor housing. The waste gas when passing through the secondary passageways (24') of the heat exchanger (12') is heated by the hot clean gas passing through the primary passageways (23') of the heat exchanger (12').

The temperature of the waste gas warmed in heat exchanger (12') can be adjusted to the desired reactor entry temperature by admixing with cold waste gas. For this purpose a shunt pipe (29') is provided, parallel with the secondary passageways (24') of the heat exchanger (12') whereby the shunt pipe (29') establishes a direct contact between supply pipe (22') and another pipe (28') which conveys the untreated gas to the top of head chamber (15'). The temperature can be regulated by changing the ratio of the warmed and cold waste gas flows with the aid of a suitable throttling member (30') placed in the stunt pipe (29').

The waste gas adjusted to the desired reactor entry temperature is introduced to the reactor housing (1') via the head chamber (15') and flows in succession through both catalytic layers (5',6') from the outside to the inside. Typically, the external layer (5') consists of a catalyst for oxidative cracking, while the internal layer (6') is a noble metal oxidation catalyst for oxidation afterburning.

After having flowed through the catalyst layers the cleaned waste gas enters via the central hole (3') in annular bottom (2') of the reactor housing into the heating chamber (25'). Burner equipment laterally joins the heating chamber (25'); the burner apparatus serves for heating the cleaned purified waste gases with the aid of a heating medium produced by burning a mixture of methane and air.

The hot pure gas flows through the primary passageways (23') of the heat exchanger (12') and acts as heating medium for the cold waste gas. The pure gas passes through the heat exchanger (12') and acts as heating medium for the cold waste gas. The pure gas passes through the heat exchanger and leaves the apparatus via the waste gas discharge (exhaust pipe) (20').

The apparatus generally operates in an autothermic manner, i.e. the exothermic procedures which take place in the catalytic layers are capable of heating the pure (clean) gas to a temperature sufficient to heat the unpurified waste gas in the heat exchanger to the desired reactor entry temperature. In this case additional heating of the heating chamber (25') becomes superfluous.

An additional heating to warm up the apparatus is generally necessary when putting the apparatus into operation and also in case of waste gases having a low contamination content.

The second apparatus disclosed above profoundly stirs the warmed waste gas in the course of the route from the heat exchanger (12') to the head chamber (15') and thus provides a highly homogenous temperature distribution of the waste gases entering the reactor housing and thereby also ensures a uniform temperature load of the catalyst layers. Local hot spots of the catalyst layers are thus considerably eliminated. Since the homogenous admixture of the heated clean gas is not a function of the heating chamber (25'), the latter can be constructed in a very compact manner.

The invention is explained in more detail in the following example which illustrate the embodiments of the invention.

EXAMPLE 1

1.1 Catalyst for zone 1 (Oxidation Cracking)

A commercially available SiO₂ carrier (Suedchemie, carrier "KA 3", balls 4-5 mm, bulk density 0.6 g/l, specific surface approximately 170 m² /g) was used as catalyst.

1.2 Catalyst for zone 2 (Oxidation Afterburning)

1000 g γ-aluminum oxide balls (Rhone-Poulenc, "SCS 250", diameter 4-6 mm, bulk density 0.71 g/l, specific surface area 253 m² /g) are precoated in an impregnation drum with 300 ml dist. H₂ O (corresponds to 2/3 of the water absorption capacity of the carrier material) and subsequently impregnated with 150 ml of a solution containing Pt and Rh. The noble metal solution contains 0.5 g 16% RhCl₃ solution (corresponds to 0.08 g Rh) and 6.4 g 25% H₂ PtCl₆ solution (corresponds to 1.6 g Pt) and is pre-neutralized with 10% Na₂ CO₃ solution in such a manner that pH 6 is obtained in the impregnation solution diluted with dist. H₂ O to 150 mi.

After the noble metal solution has been completely taken up by the porous carrier material, the catalyst is dried at 120° C. to constancy of weight and subsequently reduced 4 h at 400° C. under forming gas (95% by vol. N₂, 5% by vol. H₂). The finished catalyst contains 0.16% by wt. Pt and 0.0084 by wt. Rh.

1.3 Conversion of a test gas mixture of CO, ethylene and ethyl chloride in a laboratory reactor

A heatable tubular reactor of temperature-resistant steel is filled with 38 cm³ (corresponds to 10 cm layer thickness) of each of the two catalysts (cf. points 1.1 and 1.2) so that a double-bed arrangement is created. The catalytic combination is loaded at different gas entrance temperatures into the first catalytic zone and at a constant loading (GHSV=15,000 h⁻¹) with a test gas mixture consisting of

1% by vol. CO

0.7% by vol. ethylene

0.5% by vol. ethylene chloride remainder air and the degree of conversion achieved is measured behind the second catalytic zone. (Results cf. table 1) Temperature peaks occur thereby in the reaction zones which can be up to 300° C. above the gas entrance temperature in zone 1, depending on the catalytic combination of the examples mentioned in the following.

EXAMPLE 2

2.1 Catalyst for zone 1

Commercially available γ-aluminum oxide balls (Rhone Poulenc, "SCS 250" diameter 2.4-4 mm, bulk density 0.71 g/cm³, specific surface area 253 m² /g total pore volume 52.2 cm³ /100 g) were used as catalyst for zone 1.

2.2 Catalyst for zone 2

A drilling core (diameter 20 mm, length 120 mm) from a honeycomb-shaped carrier of cordierite (Corning, 400 cells/inch², approximate composition: 14.0% by wt. MgO, 35.4% by wt. Al₂ O₃, 49.6% by wt. SiO₂) is rinsed repeatedly with a 30% aqueous dispersion of γ-aluminum oxide and dried between the individual rinsing procedures 2 h per time at 200° C. in a stream of hot air. It is subsequently tempered at 700° C. The carrier coated with Al₂ O₃ (water absorption 14%) is then immersed into a solution with the following composition:

71.4 g H₂ PtCl₆ (25% Pt=17.85 g Pt)

22.3 g RhCl₃ (16% Rh=3.57 g Rh) in 100 ml solution

which is set to pH 6 with 10% NaOH.

After having dried, the carrier is coated with 0.3% by wt. noble metal in a weight ratio of Pt: Rh=5:1. Finally, the reduction of the separated noble metal compounds is performed in a current of hydrogen stream at a temperature of 500° C. for 1 h. The specific surface area of the catalyst finished in this manner was 24 m² /g.

2.3 Catalyst testing analogous to example 1.3

(Results cf. table 1)

EXAMPLE 3

3.1 Catalyst for zone 1

The cordierite carrier mentioned in example 2.2 is rinsed repeatedly with a 30% aqueous dispersion of γ-aluminum oxide and magnesium oxide (weight ratio Al₂ O₃ : MgO=3:1), dried between the individual rinsing procedures 2 h per time at 200° C. in a stream of hot air and then tempered at 600° C. The carrier coated in this manner (water absorption 18.6%) is subsequently immersed in a solution of 60 g Ba(NO₃)₂ in 1000 ml dist. H₂ O. After having dried at 200° C. and a 3-hour tempering at 600° C. in a current of air, the finished catalyst contains 3.5% by wt. Mg and 0.56% by wt. Ba.

3.22 Catalyst for zone 2

200 g γ-aluminum oxide balls (Rhone-Poulenc, "SCS 79", diameter 22.4-4 mm, bulk density 0.67 g/l, specific surface area 80 m² /g) are precoated in an impregnation drum with 60 ml dist. H₂ O (corresponds to 22/3 of the water absorption capacity of the carrier material) and subsequently impregnated with 30 ml of a solution containing Pd and Pt. The noble metal solution contains 7.5 g 20% H₂ PdCl₄ solution (corresponds to 1.5 g Pd) and 1.22 g 25% H₂ PtCl₆ solution (corresponds to 0.3 g Pt) and is pre-neutralized with 10% Na₂ CO₃ solution to the extent that a pH of 6 is set in the solution diluted with dist. H₂ O to 30 ml.

After drying and reduction according to example 1.2, the finished catalyst contains 0.75% by wt. Pd and 0.15% by wt. Pt.

3.3 Catalyst testing analogous to example 1.3

(Results cf. table 1)

EXAMPLE 4

4.1 Catalyst for zone 1--analogous to example 3.1

4.22 Catalyst for zone 22

The preparation of the catalyst was performed as described in example 2.2; however, an impregnation solution of the following composition was used:

71.4 g H₂ PtCl₆ (2.5% Pt=17.85 g Pt)

17.85 g H₂ PdCl₄ (20% Pd=3.57 g Pd) in 1000 ml solution.

The finished catalyst was coated with 0.3% by wt. noble metal in a weight ratio of Pt:Pd=5:1.

4.3 Catalyst testing analogous to example 1.3

(Result cf. table 1)

EXAMPLE 5

5.1 Catalyst for zone 1

100 g of the annular Al₂ O₃ extrudate "Al 3996 R" of the Harshaw company (bulk density 0.5-0.75 g/cm3, specific surface 200 m² /g, total pore volume approximately 0.69 cm³ /g) are first impregnated with 70 ml of a solution of 60 g Ni(NO₃)₂ ×6H2₀ O in dist. H₂ O and dried at 110° to constancy of weight. The carrier pre-impregnated in this manner is subsequently re-impregnated with an impregnation solution of 60 g Ni(NO₃)₂ ×6H₂ O in 24 ml dist. H₂ O, dried once again at 110° C. and finally calcined in a stream of air at a rising temperature:

15 min. at 200° C.,

15 min at 300° C.,

15 min at 400° C.,

35 min. at 500° C.

The finished catalyst contains 20% by wt. nickel.

5.2 Catalyst for zone 2

200 g of the carrier material mentioned in example 3.2 are impregnated with a solution prepared by diluting 4 g of a 25% H₂ PtCl₆ solution with dist. H₂ O to 90 ml.

After drying at 120° C. and a three-hour reduction with forming gas (95% by vol. N₂, 5% by vol. H₂) at 500° C., the finished catalyst contains 0.5% by wt. Pt.

5.3 Catalyst testing analogous to example 1.3

(Results cf. table 1)

EXAMPLE 6

6.1 Catalyst for zone 1--analogous to example 2.1

6.2 Catalyst for zone 2

200 g of the carrier material mentioned in example 3.2 are impregnated with a solution containing 5 g of a 20% H₂ PdCl₄ solution and pre-neutralized with 10% NaOH to the extent that a pH of 4 is set in the solution diluted with dist. H₂ O to 90 ml.

After drying at 120° C. and reduction with forming gas (95% by vol. N₂, 5% by vol. H₂) at 250° C., the finished catalyst contains 0.5% by wt. Pd.

6.3 Catalyst testing analogous to example 1.3 but at 10 bars absolute pressure and once, after having reached the starting temperature, without reactor heating. The exothermicity of the conversion is sufficient to maintain the reaction even without an additional heat supply. (Results cf. table 1)

EXAMPLE 7

7.1 Catalyst for zone 1

Commercially available γ-aluminum oxide pellets (Rhone-Poulenc, "SCS 250" diameter 6-8 mm, bulk density 0.69 g/cm³, specific surface 253 m² /g were used.

7.2 Catalyst for zone 2

50 kg gamma-aluminum oxide pellets (Rhone-Poulenc, "SCS 79"), diameter 2.4-4 mm, bulk density 0.69 g/l, specific surface 80 m² /g) are first precoated in a rotating impregnation drum with 15 l dist. H₂ O (corresponds to 2/3 of the water absorption capacity of the carrier material) and subsequently impregnated with 7.5 l of a solution containing Pd and Pt. The noble metal solution specifically contains 375 g 20% H₂ PdCl₄ solution (corresponds to 75 g Pd) and 300 g 25% H₂ PtCl₆ solution (corresponds to 75 g Pt). It is pre-neutralized with approximately 2.25 l 10% Na₂ CO₃ solution to pH 6 and diluted with dist. H₂ O to 7.5 l.

As soon as the noble metal solution has been completely taken up by the carrier, the catalyst is dried at 120° C. until constancy of weight and subsequently reduced 3 h at 400° C. under forming gas (95% by vol. N₂, 5% by vol. H₂). The prepared catalyst contains 0.15% by wt. Pd and 0.15% by wt. Pt.

7.3 Conversion of a waste gas from an industrial-scale production of vinyl chloride in a pilot reactor

The first zone of the annular double-bed reactor shown in FIG. 1 is filled with 60 l of the aluminum oxide balls mentioned in 7.1 and the second zone with 70 l of the noble metal catalyst prepared as described in 7.2. After the heating phase, the pilot reactor is operated with waste gas from an industrial-scale VCM system at different temperatures and throughputs.

The burner setting remains unchanged during the entire test time:

Methane: 6 Nm₃ /h

Combustion air:75 Nm₃ /h

The hot burner gases are cooled off somewhat with 320 Nm₃ /h secondary air for material reasons. A few test settings and test results are collated in table 2.

EXAMPLE 8

8.1 Catalyst for zone 1--analogous to example 7.1

8.2 Catalyst for zone 2

50 kg γ-aluminum oxide balls (Rhone-Poulenc, "SCS 250", diameter 4-6 mm, bulk density 0.71 g/l, specific surface area 253 m² /g) are precoated with 15 l dist. H₂ O and subsequently impregnated with 7.5 l of a solution containing Pt and Rh. The noble metal solution contains 25 g 16% RhCl₃ solution and 250 g 25% H₂ PtCl₆ solution, is pre-neutralized with 500 ml 10% Na₂ CO₃ solution to pH 6 and diluted with distilled water to 7.5 l.

After the noble metal solution has been completely taken up by the carrier, the catalyst is dried at 120° C. until constancy of weight and subsequently reduced 4 h at 400° C. under forming gas (95% by vol. N₂, 5% by vol. H₂). The prepared catalyst contains 0.125% by wt. Pt and 0.008% by wt. Rh.

8.3 Conversion of a waste gas from an industrial-scale production of vinyl chloride in a pilot reactor

The testing of the above-mentioned catalysts takes place according to example 7 in the annular double-bed reactor with unchanged burner setting but with dosing in of 240 Nm³ /h secondary air. Typical test settings and results are collated in table 3.

Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.

German priority application P 37 31 688.5-43 is relied on and incorporated by reference.

                                      TABLE 1                                      __________________________________________________________________________     Test conditions and residual concentrations of CO, ethylene and ethyl          chloride in the                                                                cleaned waste gas (Laboratory Tests)                                           Temperature(°C.)                                                                          Residual contents in the cleaned gas                                                              Pressure                                  before Zone 1     CO (ppm)                                                                             C.sub.2 H.sub.4 (%)                                                                  C.sub.2 H.sub.5 Cl (%)                                                                (bar)                                     __________________________________________________________________________     Example 1                                                                            300         7.1    0.6  7.6    1                                               400         3.3   <0.1  1.1    1                                         Example 2                                                                            300         17.6   4.5  12.7   1                                               600         2.7    0.1  <0.1   1                                         Example 3                                                                            300         3.0   <0.1  8.8    1                                               400         1.2   <0.1  0.2    1                                         Example 4                                                                            500         11.2   1.1  5.9    1                                               600         7.4   <0.1  1.1    1                                         Example 5                                                                            300         10.8   0.8  9.0    1                                               500         2.6   <0.1  <0.1   1                                         Example 6                                                                            300         6.7    0.2  6.8    10                                              400         3.1   <0.1  0.6    10                                              without reactor heating                                                                    29.2   1.3  9.3    10                                        __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Test conditions and composition of noxious gas and clean gas (Example          7)*) - Pilot System                                                            Test Setting                                                                          1             2             3             4                             Waste gas                                                                             300           300           300           650                           current                                                                        (Nm/h)                                                                         Temperature                                                                           before        before        before        before                        (°C.)                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                  510    650    450    600    435    575    460    675                    __________________________________________________________________________     CO     0.83   7.2 ppm                                                                               0.85   9 ppm  0.83   12 ppm 0.81   37 ppm                        Volume %.     Volume %      Volume %      Volume %                      CH.sub.4                                                                              61 ppm 2 ppm  61 ppm 23 ppm 65 ppm 30 ppm 76 ppm 3 ppm                  C.sub.2 H.sub.4                                                                       0.44   5 ppm  0.54   16 ppm 0.56   10 ppm 0.57   5 ppm                         Volume %      Volume %      Volume %      Volume %                      C.sub.2 H.sub.6                                                                       0.10   <1 ppm 0.06   5 ppm  0.09   11 ppm 0.08   <1 ppm                        Volume %      Volume %      Volume %      Volume %                      CH.sub.3 Cl                                                                           5 ppm  <1 ppm 5 ppm  <1 ppm 3 ppm  <1 ppm 4 ppm  <1 ppm                 VCM    21 ppm <1 ppm 17 ppm <1 ppm 17 ppm <1 ppm 25 ppm <1 ppm                 C.sub.2 H.sub.5 Cl                                                                    0.016  <1 ppm 0.014  <1 ppm 34 ppm <1 ppm 0.013  <1 ppm                        Volume %      Volume %                    Volume %                      1,2-EDC                                                                               2 ppm  <1 ppm 20 ppm <1 ppm 1 ppm  <1 ppm 64 ppm <1 ppm                 aro    0.01   <1 ppm 85 ppm <1 ppm 65 ppm <1 ppm 96 ppm <1 ppm                 hydrocarbons                                                                          Volume %                                                                __________________________________________________________________________     Test conditions and composition of noxious gas and clean gas (Example 7)       Test Setting                                                                          5             6             7             8                             Waste gas                                                                             650           650           1000          1000                          current                                                                        (Nm.sup.3 /h)                                                                  Temperature                                                                           before        before        before        before                        (°C.)                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                                                                          Zone 1 after Zone 2                  448    650    430    600    410    650    355    576                    __________________________________________________________________________     CO     0.84   38 ppm 0.90   29 ppm 0.83   23 ppm 0.88   25 ppm                        Volume %      Volume %      Volume %      Volume %                      CH.sub.4                                                                              65 ppm 6 ppm  64 ppm 14 ppm 53 ppm 12 ppm 58 ppm 18 ppm                 C.sub.2 H.sub.4                                                                       0.44   14 ppm 0.48   22 ppm 0.47   26 ppm 0.54   21 ppm                        Volume %      Volume %      Volume %      Volume %                      C.sub.2 H.sub.6                                                                       0.06   <1 ppm 0.06   3 ppm  0.09   <1 ppm 0.06   26 ppm                        Volume %      Volume %      Volume %      Volume %                      CH.sub.3 Cl                                                                           4 ppm  <1 ppm 3 ppm  <1 ppm 6 ppm  <1 ppm 3 ppm  <1 ppm                 VCM    21 ppm <1 ppm 21 ppm <1 ppm 22 ppm <1 ppm 22 ppm <1 ppm                 C.sub.2 H.sub.5 Cl                                                                    0.01   <1 ppm 67 ppm <1 ppm 0.018  <1 ppm 56 ppm <1 ppm                        Volume %                    Volume %                                    1,2-EDC                                                                               0.012  <1 ppm 0.017  <1 ppm 6 ppm  <1 ppm 11 ppm <1 ppm                        Volume %      Volume %                                                  aromat 99 ppm <1 ppm 0.45   <1 ppm 0.014  <1 ppm 86 ppm <1 ppm                 hydrocarbons         Volume %      Volume %                                    __________________________________________________________________________      *) The results cited were determined during a longterm test extending ove      several months. Therefore, in a direct comparison of measured results,         different deactivations stages of the catalyst are to be taken into            consideration.                                                           

                                      TABLE 3                                      __________________________________________________________________________     Test conditions and composition of noxious gas and clean gas (Example          8)*) Pilot System                                                              Time Setting                                                                            1               2               3                                     Waste gas                                                                               1100            1100            900                                   current                                                                        (Nm.sup.3 /h)                                                                  Temperature                                                                             before Zone 1                                                                           after Zone 2                                                                          before Zone 1                                                                           after Zone 2                                                                          before Zone 1                                                                           after Zone 2                 (°C.)                                                                            490      730    440      670    500      710                          __________________________________________________________________________     CO       0.87 Volume %                                                                           34 ppm 0.85 Volume %                                                                           52 ppm 0.82 Volume %                                                                           25 ppm                       CH.sub.4 63 ppm   3 ppm  94 ppm   39 ppm 73 ppm   7 ppm                        C.sub.2 H.sub.4                                                                         0.49 Volume %                                                                           2 ppm  0.45 Volume %                                                                           43 ppm 0.46 Volume %                                                                           8.5 ppm                      C.sub.2 H.sub.6                                                                         0.06 Volume %                                                                           <1 ppm 0.06 Volume %                                                                           4.2 ppm                                                                               0.05 Volume %                                                                           <1 ppm                       CH.sub.3 Cl                                                                             2.8 ppm  <1 ppm 3.3 ppm  <1 ppm --       --                           VCM      27 ppm   <1 ppm 27 ppm   <1 ppm 24 ppm   <1 ppm                       C.sub.2 H.sub.5 Cl                                                                      42 ppm   <1 ppm 43 ppm   <1 ppm 63 ppm   <1 ppm                       1,2-EDC  4 ppm    <1 ppm 62 ppm   <1 ppm 13 ppm   <1 ppm                       aromat   51 ppm   <1 ppm 63 ppm   <1 ppm 89 ppm   <1 ppm                       hydrocarbons                                                                   __________________________________________________________________________     Test conditions and composition of noxious gas and clean gas (Example 8)       Time Setting                                                                            4               5               6                                     Waste gas                                                                               900             700             700                                   current                                                                        (Nm.sup.5 /h)                                                                  Temperature                                                                             before Zone 1                                                                           after Zone 2                                                                          before Zone 1                                                                           after Zone 2                                                                          before Zone 1                                                                           after Zone 2                 (°C.)                                                                            425      650    490      670    565      735                          __________________________________________________________________________     CO       0.86 Volume %                                                                           55 ppm 0.82 Volume %                                                                           23 ppm 0.83 Volume %                                                                           15 ppm                       CH.sub.4 77 ppm   29 ppm 74 ppm   14 ppm 72 ppm   <1 ppm                       C.sub.2 H.sub.4                                                                         0.53 Volume %                                                                           39 ppm 0.49 Volume %                                                                           20 ppm 0.38 Volume %                                                                           <1 ppm                       C.sub.2 H.sub.6                                                                         0.05 Volume %                                                                           38 ppm 0.06 Volume %                                                                           1 ppm  0.06 Volume %                                                                           <1 ppm                       CH.sub.3 Cl                                                                             --       --     --       --     --       --                           VCM      24 pm    <1 ppm 57 ppm   <1 ppm 23 ppm   <1 ppm                       C.sub.2 H.sub.4 Cl                                                                      74 ppm   <1 ppm 22 ppm   <1 ppm 66 ppm   <1 ppm                       1,2-EDC  0.010 ppm                                                                               <1 ppm 1 ppm    <1 ppm 1 ppm    <1 ppm                       aromat   87 ppm   <1 ppm 78 ppm   <1 ppm 73 ppm   <1 ppm                       hydrocarbons                                                                   __________________________________________________________________________      *) The results cited were determined during a longterm test extending ove      several months. Therefore, in a direct comparison of measured results,         different deactivations stages of the catalyst are to be taken into            consideration.                                                            

We claim:
 1. An apparatus for purifying waste gases comprising a cylindrical reactor housing (1'), a heat exchanger (12'), a supply pipe (22') for supplying untreated waste gas to said heat exchanger, and an exhaust pipe (20') for conveying cleaned waste gas from said housing; said heat exchanger (12') having primary passageways (23') and secondary passageways (24') in close thermal contact;said reactor housing (1') having an upper portion forming a head chamber (15') and a lower portion forming a heating chamber! (25'), wherein said heating chamber (25') is connected via said primary passageways (23') of said heat exchanger (12') with said exhaust pipe (20'); said supply pipe (22') being connected with an inlet port (26') of said secondary passageways (24'); said head chamber (15') being connected via another pipe (28') with an outlet port (27') of said heat exchanger (12'); said secondary passageways (24') being bypassed by a shunt pipe (29') running from said supply pipe (22') directly to said another pipe (28'), wherein said shunt pipe (29') has a throttling member (30'); said reactor housing (1') including a catalytic material holding section being separated from said heating chamber (25') by an annular bottom (2') having a central hole (3') with a circumferential edge (10'), wherein the outer periphery of said annular bottom (2') is connected to said cylindrical reactor housing (1'), and wherein said catalytic material holding section is separated from said head chamber (15') by common plate (4') and outer wall member (9'); the boundaries of said catalytic material holding section being said annular bottom (2'), said outer wall member (9'), and said common plate (4'); said reactor housing (1'), including an arrangement of two coaxial annular cages constructed for holding separate catalyst layers (5', 6'); said annular cages (5',6') being formed by three conical wall members (7',8',9') including an inner wall (7'), a separating wall (8'), and said outer wall (9'); said conical wall members being perforated and resting with their bases on said annular bottom (2') of said catalytic material holding section and being closed at the top by said common plate (4'); said conical wall members (7', 8', 9') being sealed to said annular bottom (2') in a gas tight manner whereby said inner wall (7') forms a circumferential border on said annular bottom (2') which coincides with said circumferential edge (10') of said central hole (3'); said reactor housing (1') and said outer wall (9') forming an annular space (11') with increasing cross section from the bottom to the top of said catalytic material holding section due to the decreasing diameter of said outer wall (9'); said heating chamber (25') being laterally connected to a pipe (14') for the supply of a gaseous heating medium; said common plate (4') closing the central hole so as to downwardly force waste gas coming into said annular space (11') through said catalytic material holding section and through said central hole (3') via said primary passageways (23') of said heat exchanger (12') towards said exhaust pipe (20').
 2. The apparatus according to claim 1, wherein said coaxial annular cages (5', 6') contain bulk catalytic material so as to form two catalytic layers. 